The present invention relates to a flame detector.
As shown in FIG. 1, the related-art flame detector includes a first band pass filter 53a for transmitting infrared rays of, for example, about 4.4 to 4.5 μm, which are in a wavelength band of CO2 resonance radiation, a first infrared-ray receiving element (infrared-ray receiving sensor) 53b for receiving light from the first band pass filter 53a, a second band pass filter 54a for transmitting infrared rays of about 5.0 μm, which are in the vicinity of the wavelength band of CO2 resonance radiation, and a second infrared-ray receiving element (infrared-ray receiving sensor) 54b for receiving light from the second band pass filter 54a. A common protective glass 52 made of a member having infrared translucency such as sapphire glass is provided on the front surface of the first band pass filter 53a and the second band pass filter 54a. 
With such a configuration, a sensor output based on the infrared ray of about 4.4 to 4.5 μm set in the first band pass filter 53a is generated from the first infrared-ray receiving element 53b. Moreover, a sensor output based on the infrared ray of about 5.0 μm set in the second band pass filter 54a is generated from the second infrared-ray receiving element 54b. The sensor output of the first infrared-ray receiving element 53b and the sensor output of the second infrared-ray receiving element 54b are amplified by amplifiers 55 and 56, respectively, and then a comparator 57 compares the sensor outputs to detect flame.
That is, as shown in FIG. 2, a relative intensity of the flame is the highest in the wavelength band (first wavelength band) of about 4.4 to 4.5 μm set in the first band pass filter 53a, and the relative intensity of the flame is the smallest in the wavelength band (second wavelength) of about 5.0 μm set in the second band pass filter 54a. Accordingly, it is possible to detect the flame when the sensor output of the first infrared-ray receiving element 53b is larger than that of the second infrared-ray receiving element 54b. 
In this case, the sapphire glass used in the protective glass 52 transmits light of about 0.3 to 7.6 μm. That is, since the sapphire glass used in the protective glass 52 also transmits the visible light and the near-infrared rays as well as the infrared rays, the visible light and the near-infrared rays are directly emitted to the first band pass filter 53a and the second band pass filter 54a. In this case, in the first band pass filter 53a and the second band pass filter 54a, an optical energy (that is, the optical energy of the visible light and the near-infrared rays) outside the transmissive band is converted into heat, and thus is emitted as secondary radiation to the surroundings.
As shown in FIG. 1, the known flame detector has a problem in that the known flame detector may also detect the visible light and the near-infrared rays which are not originally transmitted through the band pass filters since the first infrared-ray receiving element 53b and the second infrared-ray receiving element 54b detect the secondary radiation.
In particular, in an environment where the flame detector is installed, there are factors causing a false alarm due to sunlight, an electric bulb, or the like. Accordingly, since the flame may be much affected by the visible light and the near-infrared rays emitted from the sunlight, the electric bulb, or the like, there arises a problem in that the flame cannot be detected with good accuracy.
This problem also arises in a one-wavelength type flame detector for detecting flame based on the infrared rays of about 4.4 to 4.5 μm or a multi-wavelength type flame detector.
In the past, a technique disclosed in Patent Document 1 was devised to solve this problem (that is, to prevent the influence (for example, the false alarm) caused due to the secondary radiation). That is, in Patent Document 1, a deposition film for cutting the visible light and the near-infrared rays is formed on the rear surface of the protective glass in order to prevent the visible light and the near-infrared rays from being incident on the first band pass filter and the second band pass filter.
Patent Document 1: Japanese Patent Publication No. 2006-98372A
Like Patent Document 1, when the protective glass has a function for cutting the visible light and the near-infrared rays to prevent the visible light and the near-infrared rays from being incident on the first band pass filter and the second band pass filter, the influence (for example, the false alarm) caused due to the secondary radiation can be prevented. However, there arises a problem in that sensitivity for detecting the flame may deteriorate since attenuation occurs as much as light transmitted through the protective glass, thereby reducing an amount of light transmitted through the first band pass filter and the second band pass filter.